[server] drop system_source_cnx for a simpler internal API system_source_cnx() can be replaced with source_cnx(). If system_source_cnx() is called with dbms_system_base=False, it is strictly equivalent to calling source_cnx() with the same source, privileges and interactivity. If system_source_cnx() is called with dbms_system_base=True, it is equivalent to calling source_cnx() with the system database and the same source, privileges and interactivity. No use of the default value for 'special_privs' was found in the code.

from yams.buildobjs import ComputedRelation, EntityType, RelationDefinition
from cubicweb.schema import RRQLExpression

class Subject(EntityType):
    pass

class Object(EntityType):
    pass

class relation(RelationDefinition):
    subject = 'Subject'
    object = 'Object'

class computed(ComputedRelation):
    rule = 'S relation O'
    __permissions__ = {'read': (RRQLExpression('S is ET'),)}